An important property of a semiconductor device is the breakdown voltage. There exists insulator breakdown and breakdown of p-n junctions.
The atoms in insulating materials have very tightly-bound electrons, resisting free electron flow very well. However, insulators cannot resist indefinite levels of voltage. With enough voltage applied, any insulating material will eventually succumb to the electrical “pressure” and electron flow will occur, called insulator breakdown.
Thickness of an insulating material plays a role in determining its breakdown voltage, otherwise known as dielectric strength. Specific dielectric strength is often listed in terms of MV per cm, but in practice it has been found that the relationship between breakdown voltage and thickness is not exactly linear.
Also the maximum reverse bias voltage that can be applied to a p-n diode is limited by breakdown, breakdown being characterised by the rapid increase of the current under reverse bias. The corresponding applied voltage is referred to as the breakdown voltage, i.e. that reverse voltage at which a semiconductor device changes its conductance characteristics.
The breakdown voltage is a key parameter of power devices. The breakdown of logic devices is equally important as one typically reduces the device dimensions without reducing the applied voltages, thereby increasing the internal electric field.
Furthermore, breakdown voltage is also an important parameter in trench isolation structures. In some cases, upon initial biasing of certain high voltage semiconductor devices, the devices may pass current at a voltage less than the designed breakdown voltage. Upon continued stress and passage of current, the breakdown voltage will then “walk out” to its design breakdown voltage. This unstable electrical isolation behaviour may be associated with high electric fields across conventional isolation trenches and/or avalanche breakdown at the corners of conventional trench isolation structures.
For semiconductor devices it is important that the breakdown voltage is as high as possible.
Another important property of a semiconductor device is the specific on-resistance.
The specific on-resistance of a semiconductor device is the output resistance when the device is in its fully “on” or conducting state. For semiconductor devices it is important that the specific on-resistance is as low as possible, in order to dissipate as little energy as possible.
An ideal high-voltage switch (MOSFET) should have no resistance in its “on state”, when it conducts electricity. Conversely, in its “off state”, it should block an infinitely high voltage and prevent any electrical current from flowing through it. In reality, this is impossible. Doubling the voltage blocking capability typically leads to an increase in the on-state resistance by a factor of five—a physical law often referred to as the silicon limit for performance.
In ‘Temperature characteristics of a new 100V rated power MOSFET, VLMOS (Vertical LOCOS MOS)’, proceedings of 2004 International Symposium on Power Semiconductor Devices & ICs, Kitakyushu, p. 463-466, Masahito Kodama et al. describe a 100V rated power MOSFET. Manufacturing of the device comprises making a shallow and a deep trench and using local oxidation of silicon (LOCOS) technology. A thick oxide is formed at the deep trench region and a thin oxide at the shallow trench region by an oxidation process. From device simulation, it was predicted that the VLMOS had a breakdown voltage of 113 V and a specific on-resistance of 51.3 mΩ·mm2 at a gate voltage of 20V. The silicon limit value at the breakdown voltage of 113 V is 81.4 mΩ·mm2 and thus, the specific on-resistance of the VLMOS can be less than the silicon limit.
In ‘Tunable Oxide-Bypassed Trench Gate MOSFET: Breaking the Ideal Superjunction MOSFET performance Line at Equal Column Width’, IEEE electron device letters, vol. 24(11), November 2003, Yang et al. describe a superjunction MOSFET power device which has a higher blocking capability and lower on-state resistance that breaks the conventional unipolar silicon limit. The superjunction device concept has been proven to have advantages of higher breakdown voltage and lower specific on-resistance for making unipolar devices. The tunable oxide-bypassed (TOB) structure is formed by etching deep trenches beside drift region followed by thick thermal oxide growth and heavily doped polysilicon deposition. Measurement results show a maximum breakdown voltage of 79V at 5V tuning voltage. It is also observed that the good tunable characteristic does not deteriorate leakage current. The corresponding specific on-resistance is measured to be 0.674 mΩ·mm2 at a gate voltage of 20V without application of tuning voltage.
A disadvantage of the above-described devices is, that when growing the thick oxide in the trench, a notch can be formed at the bottom of the trench. Because of this, a higher electric field will occur at this position which can result in premature breakdown of the device.